A pulsar has been found with behavior so unusual it is thought likely to be the first of a new class of neutron stars. Its 76 second period and unexpected location make astronomers think this could be the first example of ultra-long period magnetars, a type of neutron star long theorized, but never before seen.
Neutron stars are left behind by supernova explosions in stars with masses between 10 and 25 times the Sun's. Initially immensely hot, they generate no new energy, unless accreting matter onto them, and therefore gradually cool down. Consequently, while the galaxy is thought to be littered with neutron stars, we can only see those young enough to have retained much of their heat. Pulsars are a class of neutron stars that emit radio wave beams that sweep the sky as they spin.
Nature Astronomy reports the discovery of a pulsar named PSR J0901-4046 with several novel features that make it unlike anything seen before, but the details are far from settled.
Pulsars start out spinning thousands of times a second, and slow down until they have spin periods of tens of seconds. At 75.9 seconds PSR J0901-4046's period is more than three times the previous record, an anomalous white dwarf pulsar and mystery case aside.
However, that is only the beginning of its unusual features. Instead of producing identical pulses every time its radio beam sweeps past Earth, it displayed seven different types of pulses, based on shape and intensity. The most common are those the authors refer to as “split-peak”, followed by those described as “quasi-periodic” and “partially nulling”.
An additional odd feature to PSR J0901-4046 is that it was located in the Milky Way's Vela-X1 region, an area known as a neutron star “graveyard”. Any such stars there are thought to be old enough to have stopped emitting, since large star formation stopped there a long time ago.
The paper concludes PSR J0901-4046 has an age of 5.3 million years, old by pulsar standards but not exceptionally so. However, many other aspects of what has been witnessed are harder to explain.
“Ultimately, it is unclear what causes the quasi-periodicity in PSR J0901-4046,” the paper notes. “Global magneto-elastic axial (torsional) oscillations are a tempting explanation, but the persistence of our periodicities would require repeated triggers and/or very long damping times.”
One likely hypothesis is that PSR J0901-4046 is a magnetar, a rare form of pulsar marked by its immensely strong magnetic fields. However, many of its measured features are even less like other magnetars than they are like certain non-magnetar pulsars. For example, unlike other known magnetars, it has not been found to emit X-Rays.
It has previously been proposed that a class of old, ultra-long period magnetars might exist, and PSR J0901-4046 may be the first example we have found.
Crucially, it would have been very easy to miss PSR J0901-4046. "The majority of pulsar surveys do not search for periods this long, so we have no idea how many of these stars might exist,” Dr Manisha Caleb of the University of Sydney said in a statement. Consequently, it is likely there are many pulsars similar to PSR J0901-4046 in the galaxy, and we've been overlooking them.
If that's the case, either there are many more neutron stars in galaxy than we previously calculated, or we've been underestimating pulsars' life cycles, or the relationship between magnetars and other neutron stars has been misunderstood.